1. Field of the Invention
The invention pertains to an analog-digital converter that works on the principle of circuits with switched capacitors, i.e. using transfers of elementary electrical charges from input capacitors towards output capacitors.
2. Description of the Prior Art
A prior art diagram of a converter with switched capacitors is shown in FIG. 1.
This converter works under the control of a clock defining a sampling frequency F and the conversion is done in n+1 clock cycles where n is a whole number that defines the converting precision.
The circuit comprises n capacitances C.sub.o to C.sub.n-1, weighted according to a pure binary code, the lowest-value capacitor having a value of c and the other capacitors having values of 2c, 4c, ... 2.sup.n-1 c respectively; furthermore, there is another capacitor provided, with a basic value of c.
The capacitors have a common armature terminal connected to the reversing input of a differential amplifier, the other (non-reversing) input of which is connected to the ground.
The other armature terminal of each capacitor may be connected either to the ground or to a reference potential Vref or, again, to an input analog voltage Vin which it is sought to convert. Finally, the reversing input of the amplifier may be connected to the ground by a switch.
The device works as follows:
In a first stage, with the reversing input of the amplifier grounded, the potential Vin to be converted is applied to all the capacitors. Then, the input of the amplifier is disconnected from the ground, so that a total charge remains stored at the armatures terminals of the capacitors which remain connected to the input of the amplifier.
In a second stage, the highest-value capacitor is connected to the reference potential Vref, while the others are all grounded. Depending on the value of the potential Vin (with respect to Vref), the output signal of the amplifier is flipped over to negative or positive, and this flip makes it possible to connect the highest-value capacitor (weight n-1) either to the ground or to Vref throughout the remaining part of the conversion.
In a third stage, it is the capacitor having the immediately smaller value (n-2) that is connected to Vref while the lower-value capacitors are grounded; here again, the direction in which the amplifier flips determines whether this capacitor with a value of n-2 is to be connected to the ground or to Vref throughout the rest of the conversion.
In the following stages, the capacitors with decreasing values are thus connected to Vref and then grounded or left at Vref, depending on the direction of the amplifier flips.
At the end of the conversion (stage n+1), the state of the switches connecting the various capacitors to the ground or to Vref defines a binary code representing a digital value of the analog voltage Vin to be converted.
This circuit arrangement is suited to the conversion of analog voltages provided that these voltages have reference potentials at ground potential (it being recalled that the amplifier has one non-reversing input permanently connected to the ground and one reversing input connected to the ground during the first stage).
If a differential voltage without a ground reference is to be convered into digital form, this voltage must first be transformed into a non-differential voltage with respect to a ground reference potential. This transformation requires an additional differential amplifier which it would be desirable to avoid.
The present invention proposes a conversion circuit with switched capacitors capable of converting differential voltages without requiring any additional differential amplifier.
The invention comprises essentially an additional capacitor with a value substantially equal to the sum of the other weighted capacitors of the system, and capacitor switching and connecting stages of capacitors which will be described below.
3. Summary of the Invention
A system according to the invention comprises, more precisely:
An input E+ and an input E-, as well as a ground point, with an analog voltage to be converted being applied between the input E+ and the input E-.
A high-gain differential amplifier with a non-reversing input connected to the ground and a reversing input;
A first change-over switch to connect the reversing input to the ground or to disconnect the reversing input from the ground;
A series of capacitors all having terminal connected, during the conversion, to the reversing input of the amplifier;
A series of change-over switches to connect the other terminal of each capacitor, except the one having the highest value, either to a first node of the circuit or to a second node, the second node being connected to a first reference potential;
A second change-over switch to connect the first node either to the input E+ or to a second reference potential;
A third change-over switch to connect the other terminal of the highest-value capacitor either to the input E- or to a third reference potential.
Preferably, the circuit further comprises an additional capacitor with a value equal to the lowest-value capacitor of the series, the said additional capacitor having one terminal connected to the reversing input of the differential amplifier and the other terminal connected to the first node of the circuit.
In a first embodiment of the invention, the second and third reference potentials are formed by the ground. In this case, however, it is necessary to begin every conversion stage by ascertaining that the analog voltage applied between the inputs E+ and E- actually has the same sign as the potential difference between the second reference potential and the ground. The amplifier may be used to make this preliminary comparison and, if necessary, to command a revresal of polarity of the analog voltage applied between the inputs E+ and E-.
It is preferable then to modify the circuit by providing an additional capacitor with a value which is half that of the lowest-value capacitor of the series. This additional capacitor is switched over between the first reference potential and the ground.
In another embodiment, the first reference potential is a positive potential +Vref, the second reference potential is a negative potential -Vref with the same value as the first potential, and the third potential is the ground.